# Velocity Voxel Scale

Velocity Voxel Scale is a new parameter, introduced in Houdini 19.5. The parameter allows for separate definitions for volume and and velocity voxels in pyro simulations. Depending on the parameter settings, you will observe a significant change in simulation time.

• Velocity Voxel Scale is available for DOP and SOP pyro networks.

• For DOP setups, the Velocity Voxel Scale parameter is on the Smoke Object (Sparse) node under Properties.

• For SOP setups, you can find the settings in the Pyro Solver under Setup ▸ General.

Velocity Voxel Scale is a multiplier and applied on top of the Voxel Scale parameter. Higher values create a coarser grid with bigger cells. This results in faster simulations, but you will also lose detail on the motion. So you could have Voxel Size at `0.1` and Velocity Voxel Scale at `2` to get a simulation where every field uses `0.1`, except velocity will be at `0.1*2=0.2`. This way, you have lots of voxels in the visible fields so they look sharp, but fewer velocity voxels where a lot of the expensive calculations happen. The caveat is that you have fewer degrees of freedom in the velocity. The motion will be less detailed compared to doing a simulation at full resolution.

To give you some figures and facts on simulation times and results, please take a look at the images.

1. The first simulation uses `0.1 x 1`. Both grid scales are equal. Simulation time is set as the baseline (1.0).

2. The second parameter combination is `0.1 x 2`. The cell size of the velocity grid is two times bigger. The simulation was 1.6× faster.

3. In the third image, scales are `0.2 x 1`. Again, both grid scales are equal, but compared to the first simulation, cooking time was 7.5× faster.

# Pyro

## Legacy Pyro ¶

• Creates a clustered (partitioned) pyro simulation with multiple containers, using the selected point geometry as sources. This is useful for large or sparse sources, and for distributing simulations to a farm.